1. Field of the Invention
The present invention relates to nephelometers and, more particularly, to a nephelometric detection system for measuring scattered light free of undesired light reflections.
2. Description of the Prior Art
Copending U.S. patent application Ser. No. 796,621, (Anderson et al), assigned to the assignee of the present invention, discloses a system for the nephelometric assay of antigens and antibodies. For this purpose, an antigen-antibody precipitate forming reaction is conducted in an optically transparent sample container or vial, preferably a cylindrical glass vial. An excitation system directs a beam of light into the sample container and a detection system measures light scattered at a forward angle by the precipitate. As set forth in detail in the application, the scattered light signal provides a quantitative measure of the desired antigen or antibody reaction component and additionally provides an indication of which of the two components is present in excess.
In the foregoing nephelometric system, the detection system for viewing light scattered within the sample container includes a light pipe aligned with a window area of the container to intercept light exiting through the window area. While reasonably accurate measurements can be made with such an arrangement, it has been found that a background signal of reflected light is superimposed on the scattered light signal measured by the detection system. The background signal component results from multiple reflections of the excitation beam within the sample container while entering and exiting the container. Since the sample container is cylindrical and formed of glass, it is theorized that the cylindrical air-glass and liquid-glass interfaces produce the undesirable reflections. Whatever the cause of the internal reflections, however, the detection system intercepts many of these reflections thereby superimposing these as a background signal on the desired scattered light signal. As a result the output scatter signal exhibits a relatively low signal-to-background ratio.
In another prior approach, the detection system includes a lens assembly focused at the center of the sample container. While such reduces the background signal to some degree, the background reflections are still intercepted in sufficient quantity to degrade the scattered light signal.